Abstract:

A method for driving a liquid drop ejector (1) equipped with a
piezoelectric actuator (7) including a piezoelectric ceramic layer (6)
having a size covering a plurality of pressurizing chambers (2). An
arbitrary piezoelectric deformation region (8) of the liquid drop ejector
(1) is deflected in one thickness direction and the opposite direction,
respectively, by applying a driving voltage waveform including a first
voltage (-VL) and an equivalent second voltage (+VL) of the
opposite polarity in order to vary the volume of the pressurizing
chambers (2) of a corresponding liquid drop ejecting portion (4), and a
liquid drop is ejected through a communicating nozzle (3). Since gradual
creep deformation of the inactive region (16) of the piezoelectric
ceramic layer (6) is prevented, the ink drop ejection performance is
maintained at a good level over a long term.

Claims:

1. A method for driving a liquid ejector that comprises(A) a substrate
formed by arranging a plurality of liquid droplet ejecting portions each
having a pressurizing chamber to be filled with a liquid and a nozzle
communicating with the pressurizing chamber for ejecting the liquid from
the pressurizing chamber as a liquid droplet in a plane direction; and(B)
a plate-shaped piezoelectric actuator laminated on the substrate
including at least one piezoelectric ceramic layer having a size covering
a plurality of a pressurizing chambers of the substrate,while the
piezoelectric actuator is divided into a plurality of piezoelectric
deformation regions arranged correspondingly to the respective
pressurizing chambers and individually deflected in a thickness direction
by individual voltage application and a restricted region surrounding the
piezoelectric deformation regions, characterized that:a driving voltage
waveform including a first voltage and a second voltage equivalent to the
first voltage and opposite in polarity thereto is applied to an arbitrary
piezoelectric deformation region of the piezoelectric actuator of the
liquid ejector, for deflecting the piezoelectric deformation region in
one thickness direction and the opposite direction each and varying a
volume of the pressurizing chamber of the corresponding liquid droplet
ejecting portion to eject a liquid droplet through the nozzle
communicating with the pressurizing chamber.

2. The method for driving a liquid ejector according to claim 1,
whereinthe piezoelectric ceramic layer is made of a PZT-type
piezoelectric ceramic material and divided into an active region
corresponding to the piezoelectric deformation region and an inactive
region corresponding to the restricted region, while the C-axis
orientation IC of the ceramic material obtained from the intensity
I.sub.(200) of a diffraction peak of the [200] plane and the intensity
I.sub.(002) of a diffraction peak of the [002] plane in an X-ray
diffraction spectrum by the following expression
(1):IC=I.sub.(002)/I.sub.(002)+I.sub.(200)) (1)is kept in the
range of 1 to 1.1 times as that in an undriven initial state after
driving.

3. The method for driving a liquid ejector according to claim 1, wherein
an area of a P-E hysteresis loop showing the relation between the
intensity of electric field E (kV/cm) and the polarization quantity P
(μC/cm2) of the piezoelectric ceramic layer in driving by
applying the driving voltage waveform to the piezoelectric deformation
region of the piezoelectric actuator is set to not more than 1.3 times of
an area of a P-E hysteresis loop in driving by applying a driving voltage
waveform on-off controlling a single polarity voltage having a value
twice of the value of the first and second voltages of the driving
voltage waveform to the piezoelectric deformation region.

4. The method for driving a liquid ejector according to claim 1, wherein
the first and second voltages are set to such a value that the intensity
of electric field E (kV/cm) of the piezoelectric deformation region of
the piezoelectric actuator is not more than 0.8 times of the intensity of
a coercive electric field Ec of the piezoelectric ceramic layer.

5. The method for driving a liquid ejector according to claim 1, wherein a
state is maintained applying no voltage to the piezoelectric deformation
region in a standby state not ejecting liquid droplets.

6. The method for driving a liquid ejector according to claim 1,
whereinthe piezoelectric actuator comprises:(i) a single piezoelectric
ceramic layer divided into an active region corresponding to a
piezoelectric deformation region expanded/contracted in the plane
direction by voltage application in the thickness direction and an
inactive region corresponding to the restricted region; and(ii) a
oscillator plate laminated on one side of the piezoelectric ceramic layer
and deflected in the thickness direction due to the expansion/contraction
of the active region in the plane direction, andthe piezoelectric
deformation region of the piezoelectric actuator is vibrated in the
thickness direction by applying the driving voltage waveform to the
active region of the piezoelectric ceramic layer and
expanding/contracting the active region in the plane direction.

7. The method for driving a liquid ejector according to claim 1,
whereinthe piezoelectric actuator comprises:(I) a first piezoelectric
ceramic layer divided into an active region corresponding to a
piezoelectric deformation region expanded/contracted in the plane
direction by voltage application in the thickness direction and an
inactive region corresponding to the restricted region; and(II) a second
piezoelectric ceramic layer laminated on one side of the first
piezoelectric ceramic layer and expanded/contracted in the plane
direction by voltage application in the thickness direction, andthe
piezoelectric deformation region of the piezoelectric actuator is
vibrated in the thickness direction by expanding/contracting the second
piezoelectric ceramic layer in antiphase with expansion/contraction of
the active region synchronously with application of the driving voltage
waveform to the active region of the first piezoelectric ceramic layer
for expanding/contracting the active region in the plane direction.

8. The method for driving a liquid ejector according to claim 1,
whereinthe piezoelectric actuator comprises a single piezoelectric
ceramic layer divided into an active region corresponding to the
piezoelectric deformation region deflected in the thickness direction by
voltage application and an inactive region corresponding to the
restricted region, and the piezoelectric deformation region of the
piezoelectric actuator is vibrated in the thickness direction by applying
the driving voltage waveform to the piezoelectric ceramic layer.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a method for driving a liquid
ejector.

PRIOR ART

[0002]FIG. 2 is a sectional view showing an example of a liquid ejector 1
employed for an on-demand ink jet printer or the like. FIG. 3 is a
sectional view showing a principal part of the example of the liquid
ejector 1 in an enlarged manner. Referring to FIGS. 2 and 3, the liquid
ejector 1 of this example includes a substrate 5 formed by arranging a
plurality of liquid droplet ejecting portions 4 having pressurizing
chambers 2 to be filled with ink and nozzles 3 for ejecting the ink
from'the pressurizing chambers 2 as ink droplets in the plane direction
and a plate-shaped piezoelectric actuator 7, including a piezoelectric
ceramic layer 6 having a size covering the plurality of pressurizing
chambers 2 of the substrate 5, laminated on the substrate 5.

[0003]The piezoelectric actuator 7 is divided into a plurality of
piezoelectric deformation regions 8 arranged correspondingly to the
respective pressurizing chambers 2 and individually deflected in the
thickness direction by individual voltage application and a restricted
region 9 arranged to surround the piezoelectric deformation regions 8 and
fixed to the substrate 5 to be prevented from deformation.

[0004]The piezoelectric actuator 7 of the example shown in the figures has
a so-called unimorphic structure including individual electrodes 10
individually formed on the upper surface of the piezoelectric ceramic
layer 6 in both figures correspondingly to the respective pressuring
chambers 2 for defining the piezoelectric deformation regions 8 as well
as a common electrode 11 and a oscillator plate 12 successively laminated
on the lower surface of the piezoelectric ceramic layer 6 each having a
size covering the plurality of pressurizing chambers 2. The individual
electrodes 10 and the common electrode 11 are separately connected to a
driving circuit 13, and the driving circuit 13 is connected to control
unit 14.

[0005]The piezoelectric ceramic layer 6 is made of a piezoelectric
material such as PZT, for example, and previously polarized in the
thickness direction to have piezoelectric deformation properties of a
so-called transverse vibration mode. When the driving circuit 13 is
driven by a control signal from the control unit 14 and a voltage of the
same direction as the direction of the polarization is applied between an
arbitrary individual electrode 10 and the common electrode 11, an active
region 15 corresponding to the piezoelectric deformation region 8
sandwiched between these electrodes 10 and 11 is contracted in the layer
plane direction, as shown by white lateral arrows in FIG. 3.

[0006]However, the lower surface of the piezoelectric ceramic layer 6 is
fixed to the oscillator plate 12 through the common electrode 11.
Therefore, the piezoelectric deformation region 8 of the piezoelectric
actuator 7 is deflected in accordance with the contraction of the active
region 15 to protrude in the direction of the pressurizing chamber 2 as
shown by a white downward arrow in FIG. 3 and to vibrate the ink filled
into the pressurizing chamber 2, so that the ink pressurized by this
vibration is ejected through a nozzle 3 as ink droplet.

[0007]As described in Patent Document 1, a so-called pull-push driving
method is widely and generally employed in the liquid ejector. FIG. 11 is
a graph showing the relation between an example of a driving voltage
waveform (shown by thick one-dot chain lines) of a driving voltage
VP applied to the active region 15 of the piezoelectric ceramic
layer 6 for driving the liquid ejector 1 shown in FIG. 2 by the pull-push
driving method and changes [shown by a thick solid line, (+) denotes the
distal end of the nozzle 3, i.e., the ink droplet ejection side, and (-)
denotes the side of the pressurizing chamber 2] in the volume velocity of
the ink in the nozzle 3 upon application of this driving voltage waveform
in a simplified manner.

[0008]FIG. 12 is a graph showing the relation between the example of the
driving voltage waveform (shown by thick one-dot chain lines) of the
driving voltage Vp applied to the active region 15 of the
piezoelectric ceramic layer 6 for driving the liquid ejector 1 shown in
FIG. 2 by the pull-push driving method and displacements [shown by a
thick solid line, (-) denotes the direction of the pressurizing chamber 2
(direction reducing the volume of the pressurizing chamber 2) and (+)
denotes the direction opposite to the direction of the pressurizing
chamber (direction increasing the volume of the pressurizing chamber 2)]
of the piezoelectric deformation region 8 of the piezoelectric actuator 7
upon application of this driving voltage waveform in a simplified manner.

[0009]Referring to FIGS. 2, 3 and 11, in a standby state on the left side
of t1 in FIG. 11 not ejecting ink droplets from the nozzle 3, the
driving voltage V2 is maintained at VH (VP=VH) and
the active region 15 is continuously contracted in the plane direction.
Thus, the piezoelectric deformation region 8 is deflected so as to
protrude in the direction of the pressurizing chamber 2 to keep the
volume of the pressurizing chamber 2 reduced, while the ink remains in a
stationary state, i.e., the volume velocity of the ink in the nozzle 3 is
maintained at 0, and an ink meniscus formed in the nozzle 3 by the
surface tension of the ink remains stationary.

[0010]In order to eject ink droplets from the nozzle 3 and form a dot on a
sheet surface, the driving voltage Vp applied to the active region
15 is discharged (VP=0) at the preceding time t1 for releasing
the active region 15 from the contraction in the plane direction, thereby
releasing the piezoelectric deformation region 8 from the deflection.
Thus, the volume of the pressurizing chamber 2 is increased by a certain
amount, whereby the ink meniscus in the nozzle 3 is drawn into the
pressurizing chamber 2 by this increment of the volume. At this time, the
volume velocity of the ink in the nozzle 3 is temporarily increased
toward the (-) side and thereafter gradually reduced to finally approach
0, as shown in the portion between t1 and t2 in FIG. 11. This
corresponds to generally a half cycle of the natural vibration cycle
T1 of the volume velocity of the ink shown by the thick solid line.

[0011]At the time t2 when the volume velocity of the ink in the
nozzle 3 infinitely approaches 0, the driving voltage VP is charged
to VH (VP=VH) again for contracting the active region 15
in the plane direction, thereby deflecting the piezoelectric deformation
region 8. Thus, the volume of the pressurizing chamber 2 is reduced due
to the deflection of the piezoelectric deformation region 8 so that the
pressure of the ink extruded from the pressurizing chamber 2 is applied
to the ink in the nozzle 3 going to return in the direction of the distal
end of the nozzle 3 contrarily to the state where the ink meniscus is
most remarkably drawn into the pressurizing chamber 2 (the state where
the volume velocity is 0 at the time t2). Consequently, the ink in
the nozzle 3 is accelerated in the direction of the distal end of the
nozzle 3 to remarkably protrude outward from the nozzle 3.

[0012]At this time, the volume velocity of the ink in the nozzle 3 is
temporarily increased toward the (+) side and thereafter gradually
reduced to finally approach 0, as shown in the portion between t2
and t3 in FIG. 11. The ink protruding outward from the nozzle 3
seems generally cylindrical, whereby the protruding ink is referred to as
an ink column in general.

[0013]At the time (t3 in FIG. 11) when the volume velocity of the ink
protruding outward from the nozzle 3 infinitely approaches 0, the driving
voltage VP is discharged (VP=0) again for releasing the active
region 15 from the contraction in the plane direction, thereby releasing
the piezoelectric deformation region 8 from the deflection. Thus, a
negative pressure formed by releasing the piezoelectric deformation
region 8 from the deflection and increasing the volume of the
pressurizing chamber 2 again is applied to the ink going to return into
the pressurizing chamber 2 contrarily to the state most remarkably
protruding outward of the nozzle 3 (the state where the volume velocity
is 0 at the time t3). Consequently, the ink column extending from
the nozzle 3 to the utmost is cut off to form a first ink droplet.

[0014]After the ink column is cut off, the ink in the nozzle 3 is drawn
into the pressurizing chamber 2 again. At this time, the volume velocity
of the ink in the nozzle 3 is temporarily increased toward the (-) side
and thereafter gradually reduced to finally approach 0, as shown in the
portion between t3 and T4 in FIG. 11. This corresponds to
generally a half cycle of the natural vibration cycle T1 of the
volume velocity of the ink, as hereinabove described.

[0015]At the time t4 when the volume velocity of the ink in the
nozzle 3 infinitely approaches 0, the driving voltage VP is charged
to VH(V2=VH) again for contracting the active region 15 in
the plane direction, thereby deflecting the piezoelectric deformation
region 8. Thus, the ink remarkably protrudes outward from the nozzle 3
again to form an ink column, due to the same mechanism as that of the
aforementioned behavior of the ink between the times t2 and t3.
At this time, the volume velocity of the ink in the nozzle 3 is
temporarily increased toward the (+) side and thereafter gradually
reduced to finally approach 0, as shown in the portion between t4
and t5 in FIG.

[0016]11.

[0017]After the time (t5 in FIG. 11) when the volume velocity of the
ink in the nozzle 3 reaches 0, the speed of vibration of the ink is
directed toward the pressurizing chamber 2, whereby the ink column
extending from the nozzle 3 to the utmost is cut off to form a second ink
droplet. The first and second ink droplets thus formed in this manner
spatter onto the sheet surface opposed to the distal end of the nozzle 3
individually to form one dot.

[0018]The series of operations correspond to application of the driving
voltage VP having the driving voltage waveform including two pulses
each having a pulse width T2 of about half of the natural vibration
cycle T1 to the active region 15, as shown by the thick one-dot
chain lines in FIG. 11. In order to form one dot with only one ink
droplet, the driving voltage waveform may include only one pulse. In
order to form one dot with not less than three ink droplets, the pulse
may be generated by the frequency corresponding to the number of the ink
droplets. Patent Document 1: Japanese Unexamined Patent Publication No.
02-102947 A (1990)

DISCLOSURE OF THE INVENTION

Problems to be Solved

[0019]In order to drive the liquid ejector 1 having the unimorphic
piezoelectric actuator 7 shown in FIGS. 2 and 3 by the pull-push driving
method, the active region 15 of the piezoelectric ceramic layer 6 must be
continuously contracted in the plane direction in the standby state not
ejecting ink droplets from the nozzle 3 as hereinabove described.
Accordingly, an inactive region 16 of the piezoelectric ceramic layer 6
surrounding the active region 15 is continuously expanded by tensile
stress in directions shown by black arrows in FIG. 3 over a long period
in the standby state due to the contraction of the active region 15 in
the plane direction.

[0020]The inactive region 16 is gradually creep-deformed due to the domain
rotating therein to relax the stress as the time of the expansion
resulting from the tensile stress is increased. As a result, the active
region 15 released from the contraction has a high degree of potential
that it cannot be expanded up to the original stationary state due to
compressive stress received from the creep-deformed inactive region 16.
In the piezoelectric deformation region 8 of the piezoelectric actuator
7, therefore, the displacement in the thickness direction between the
state deflected in the direction shown by the white downward arrow in
FIG. 3 and the stationary state released from this deflection is
gradually reduced to cause a problem of reduction in the ink droplet
ejection performance.

[0021]In the pull-push driving method, further, a noise is caused in the
vibration of the displacement of the piezoelectric deformation region 8
as shown by a thick solid line in FIG. 12 when the driving voltage
VP applied to the active region 15 is discharged (VP=0) for
driving the piezoelectric deformation region 8 of the piezoelectric
actuator 7. The vibration of this noise (noise vibration) is added to the
aforementioned vibration of the ink resulting in a problem to destabilize
the ejection of ink droplets from the nozzle 3.

[0022]In addition, the piezoelectric actuator 7 of the unimorphic type or
the like having the piezoelectric ceramic layer 6 integrally formed in
the size covering the plurality of pressurizing chambers 2 easily causes
a so-called crosstalk transmitting the noise vibration also to other
adjacent piezoelectric deformation region 8 provided on the piezoelectric
actuator 7 when the crosstalk arises, there also lies a problem that the
ejection of ink droplets from the nozzle 3 corresponding to the other
piezoelectric deformation region 8 is destabilized.

[0023]The reason of causing the noise vibration may be attributed as
follows: the displacement of the deflection is remarkable and elastic
energy is remarkably stored in the standby state continuously applying
the driving voltage VP to the active region 15 and continuously
deflecting the piezoelectric deformation region 8 in the thickness
direction; the piezoelectric deformation region 8 shifts at a stroke from
the deflected state to a free vibratory state not constrained in shape by
the applied voltage at a stretch when the driving voltage VP is
discharged (VP=0) in order to drive the piezoelectric deformation
region 8; and the like.

[0024]These problems arise not only in the unimorphic piezoelectric
actuator but also in a bimorphic piezoelectric actuator
expanding/contracting two piezoelectric ceramic layers having
piezoelectric deformation properties of the transverse vibration mode in
opposite directions thereby entirely deflecting the same in the thickness
direction and in a monomorphic piezoelectric actuator deflecting a single
piezoelectric ceramic layer in the thickness direction without laminating
a oscillator plate thereon by preparing the same from a gradient function
material or by utilizing a semiconductor effect, so far as each of the
piezoelectric ceramic layers is integrally formed in a size covering a
plurality of pressurizing chambers.

[0025]Further, the piezoelectric ceramic layer must inevitably be
integrally formed in the size covering the plurality of pressurizing
chambers in order to further refine the liquid ejector as compared with
the present structure correspondingly to refinement of the dot pitch
associated with improvement in the picture quality of the ink jet printer
and in order to manufacture the same with excellent productivity through
the minimum number of steps. As a result, techniques are required for
preventing gradual creep deformation of the inactive region surrounding
the active regions and preventing occurrence of noise vibration
destabilizing the ejection of ink droplets in driving state of the
piezoelectric deformation region.

[0026]An object of the present invention is to provide a method for
driving a liquid ejector including a piezoelectric actuator including a
piezoelectric ceramic layer having a size covering a plurality of
pressurizing chambers, capable of maintaining the ink droplet ejection
performance at an excellent level over a long period by preventing
gradual creep deformation of an inactive region of the piezoelectric
ceramic layer and preventing occurrence of noise vibration destabilizing
ejection of ink droplets in driving of a piezoelectric deformation
region.

Solutions to the Problems

[0027]The invention according to claim 1 provides a method for driving a
liquid ejector that comprises:

[0028](A) a substrate formed by arranging a plurality of liquid droplet
ejecting portions each having a pressurizing chamber to be filled with a
liquid and a nozzle communicating with the pressurizing chamber for
ejecting the liquid from the pressurizing chamber as a liquid droplet in
a plane direction; and

[0029](B) a plate-shaped piezoelectric actuator laminated on the substrate
including at least one piezoelectric ceramic layer having a size covering
a plurality of pressurizing chambers of the substrate,

[0030]while the piezoelectric actuator is divided into a plurality of
piezoelectric deformation regions arranged correspondingly to the
respective pressurizing chambers and individually deflected in a
thickness direction by individual voltage application and a restricted
region surrounding the piezoelectric deformation regions, characterized
that:

[0031]a driving voltage waveform including a first voltage and a second
voltage equivalent to the first voltage and opposite in polarity thereto
is applied to an arbitrary piezoelectric deformation region of the
piezoelectric actuator of the liquid ejector, for deflecting the
piezoelectric deformation region in one thickness direction and the
opposite direction each and varying a volume of the pressurizing chamber
of the corresponding liquid droplet ejecting portion to eject a liquid
droplet through the nozzle communicating with the pressurizing chamber.

[0032]The invention according to claim 2 is the method for driving a
liquid ejector according to claim 1, the piezoelectric ceramic layer is
made of a PZT-type piezoelectric ceramic material and divided into an
active region corresponding to the piezoelectric deformation regions and
an inactive region corresponding to the restricted region, while the
C-axis orientation IC of the ceramic material obtained from the
intensity I.sub.(200) of a diffraction peak of the [200] plane and the
intensity I.sub.(002) of a diffraction peak of the [002] plane in an
X-ray diffraction spectrum by the following expression (1):

IC=I.sub.(002)/I.sub.(002)+I.sub.(200)) (1)

is kept in the range of 1 to 1.1 times as that in an undriven initial
state after driving.

[0033]The invention according to claim 3 is the method for driving a
liquid ejector according to claim 1 or 2, wherein an area of a P-E
hysteresis loop showing the relation between the intensity of electric
field E (kV/cm) and the polarization quantity P (μC/cm2) of the
piezoelectric ceramic layer in driving by applying the driving voltage
waveform to the piezoelectric deformation region of the piezoelectric
actuator is set to not more than 1.3 times of an area of a P-E hysteresis
loop in driving by applying a driving voltage waveform on-off controlling
a single polarity voltage having a value twice of the value of the first
and second voltages of the driving voltage waveform to the piezoelectric
deformation region.

[0034]The invention according to claim 4 is the method for driving a
liquid ejector according to any one of claims 1 to 3, wherein the first
and second voltages are set to such a value that the intensity of
electric field E (kV/cm) of the piezoelectric deformation region of the
piezoelectric actuator is not more than 0.8 times of the intensity of a
coercive electric field Ec of the piezoelectric ceramic layer. The
invention according to claim 5 is the method for driving a liquid ejector
according to any one of claims 1 to 4, wherein a state is maintained
applying no voltage to the piezoelectric deformation region in a standby
state not ejecting liquid droplets.

[0035]The invention according to claim 6 is the method for driving a
liquid ejector according to any one of claims 1 to 5, wherein the
piezoelectric actuator comprises:

[0036](i) a single piezoelectric ceramic layer divided into an active
region corresponding to a piezoelectric deformation region
expanded/contracted in the plane direction by voltage application in the
thickness direction and an inactive region corresponding to the
restricted region; and

[0037](ii) a oscillator plate laminated on one side of the piezoelectric
ceramic layer and deflected in the thickness direction due to the
expansion/contraction of the active region in the plane direction, and

[0038]the piezoelectric deformation region of the piezoelectric actuator
is vibrated in the thickness direction by applying the driving voltage
waveform to the active region of the piezoelectric ceramic layer and
expanding/contracting the active region in the plane direction.

[0039]The invention according to claim 7 is the method for driving a
liquid ejector according to any one of claims 1 to 5, wherein the
piezoelectric actuator comprises:

[0040](I) a first piezoelectric ceramic layer divided into an active
region corresponding to a piezoelectric deformation region
expanded/contracted in the plane direction by voltage application in the
thickness direction and an inactive region corresponding to the
restricted region; and

[0041](II) a second piezoelectric ceramic layer laminated on one side of
the first piezoelectric ceramic layer and expanded/contracted in the
plane direction by voltage application in the thickness direction, and

[0042]the piezoelectric deformation region of the piezoelectric actuator
is vibrated in the thickness direction by expanding/contracting the
second piezoelectric ceramic layer in antiphase with
expansion/contraction of the active region synchronously with application
of the driving voltage waveform to the active region of the first
piezoelectric ceramic layer for expanding/contracting the active region
in the plane direction.

[0043]The invention according to claim 8 is the method for driving a
liquid ejector according to claim 1 or 2, wherein the piezoelectric
actuator includes a single piezoelectric ceramic layer divided into an
active region corresponding to the piezoelectric deformation region
deflected in the thickness direction by voltage application and an
inactive region corresponding to the restricted region, and the
piezoelectric deformation region of the piezoelectric actuator is
vibrated in the thickness direction by applying the driving voltage
waveform to the piezoelectric ceramic layer.

Effect of the Invention

[0044]In the invention according to claim 1, the piezoelectric deformation
region of the piezoelectric actuator is deflected in one thickness
direction and the opposite direction individually and vibrated by
applying the driving voltage waveform including the first voltage and the
second voltage opposite in polarity to the first voltage and equivalent
thereto. Therefore, in a unimorphic piezoelectric actuator, for example,
the active region of the piezoelectric ceramic layer can be not only
contracted in the plane direction and released from the contraction
similarly to the conventional one but also expanded in the plane
direction in ejection of an ink droplet and compressive stress can be
applied to the inactive region surrounding the active region. As a
result, the inactive region can be prevented from gradual creep
deformation resulting in conventional one-sided expansion in the plane
direction.

[0045]This also applies to other types of piezoelectric actuators. In a
conventional bimorphic piezoelectric actuator, for example, an active
region of a single piezoelectric ceramic layer (referred to as a first
piezoelectric ceramic layer) must be continuously contracted in the plane
direction while an active region of the other piezoelectric ceramic layer
(referred to as a second piezoelectric ceramic layer) must be
continuously expanded in the plane direction in a standby state. As a
result, the respective inactive regions is gradually creep-deformed to be
expanded in the plane direction in the first piezoelectric ceramic layer
and to be contracted in the plane direction in the second piezoelectric
ceramic layer.

[0046]According to the driving method of the invention in claim 1,
however, the active region of the first piezoelectric ceramic layer is
expanded in the plane direction so that compressive stress can be applied
to the inactive region surrounding the active region while the active
region of the second piezoelectric ceramic layer is contracted in the
plane direction so that tensile stress can be applied to the inactive
region surrounding the active region. Thus, the inactive regions around
the respective active regions can be prevented from gradual creep
deformation.

[0047]In a conventional monomorphic piezoelectric actuator, on the other
hand, an active region of a piezoelectric ceramic layer is continuously
deflected in one layer thickness direction in a standby state. As a
result, an inactive region is gradually creep-deformed so that an area of
the inactive region in the thickness direction corresponding to the
protruding side of the active region is compressed in the plane direction
and an opposite area is expanded in the plane direction. In the driving
method according to claim 1 of the present invention, however, the
piezoelectric ceramic layer is deflected also in the direction opposite
to thickness direction so that tensile stress can be applied to the area
of the inactive region in the thickness direction corresponding to the
protruding side of the active region and compressive stress can be
applied to an opposite area. Accordingly, the inactive region around the
active region can be prevented from gradual creep deformation.

[0048]According to the driving method of the invention in claim 1, the
displacement of the deflected piezoelectric deformation region in the
thickness direction with respect to a stationary state not subjected to
voltage application can also be reduced as compared with the conventional
one. Assuming that the displacement in the thickness direction between
the stationary state and the deflected state is 1 in a conventional
driving method deflecting the piezoelectric deformation region of the
piezoelectric actuator only in one direction, for example, the
displacements for deflecting the piezoelectric deformation region in one
thickness direction and the opposite direction for setting the total
displacement of the piezoelectric deformation region of the piezoelectric
actuator in the thickness direction identically to 1 can be each
generally halved in the driving method according to claim 1 of the
present invention. Therefore, the tensile stress applied to the inactive
region of the piezoelectric ceramic layer can be reduced when the
piezoelectric deformation region is deflected, whereby the inactive
region can be further reliably prevented from gradual creep deformation.

[0049]According to the driving method of the invention in claim 1,
further, it is also possible to suppress occurrence of noise vibration
destabilizing ejection of ink droplets caused in the conventional
pull-push driving method in driving of the piezoelectric deformation
region of the piezoelectric actuator. In other words, the displacement of
the deflection of the piezoelectric deformation in the standby state can
be reduced as compared with the conventional one in the driving method
according to claim 1 of the present invention as hereinabove described,
whereby storage of elastic energy can be reduced.

[0050]Further, the piezoelectric deformation region can be constrained in
shape in the state deflected in the thickness direction by the voltage
application in the standby state and can be constrained in shape in the
state deflected in the opposite direction by application of the voltage
opposite in polarity in a driving state. Accordingly, occurrence of noise
vibration can be suppressed in each state.

[0051]Therefore, destabilization of ejection of ink droplets from the
nozzle corresponding to the piezoelectric deformation region as well as
destabilization of ejection of ink droplets from the nozzle corresponding
to an adjacent piezoelectric deformation region resulting from occurrence
of a crosstalk can be reliably prevented by suppressing occurrence of
noise vibration in vibration of the displacement of the piezoelectric
deformation region in the driving state.

[0052]According to the driving method of the invention in claim 1,
therefore, the ink droplet ejection performance can be maintained at an
excellent level over a long period by preventing gradual creep
deformation of the inactive region of the piezoelectric ceramic layer
having the size covering the plurality of pressurizing chambers included
in the piezoelectric actuator of the liquid ejector and preventing
destabilization of ejection of ink droplets resulting from noise
vibration caused in the driving state of the piezoelectric deformation
region.

[0053]According to the driving method of the invention in claim 1,
further, creep deformation of the inactive region of the piezoelectric
ceramic layer can be prevented as hereinabove described. As a result, the
crystalline state of the inactive region can be prevented from changing.
In addition, the crystalline state of the active region can also be
prevented from changing due to compressive stress received from the
creep-deformed inactive region. Therefore, the crystalline states of both
regions of the piezoelectric ceramic layer can be maintained in the
initial states.

[0054]When the piezoelectric ceramic layer is made of a PZT-type
piezoelectric ceramic material, as mentioned in claim 2, for example,
both of the crystalline states of the active region and the inactive
region can be so maintained that the C-axis orientation IC showing
the crystalline state of the ceramic material obtained from the intensity
I.sub.(200) of the diffraction peak of the [200] plane and the intensity
I.sub.(002) of the diffraction peak of the [002] plane in the X-ray
diffraction spectrum by the following expression (1):

IC=I.sub.(002)/(I.sub.(002)+I.sub.(200)) (1)

is kept in the range of 1 to 1.1 times as that in the undriven initial
state after driving.

[0055]According to the driving method of the invention in claim 3, the
area of the P-E hysteresis loop showing the relation between the
intensity of electric field E (kV/cm) and the polarization quantity P
(μC/cm2) of the piezoelectric ceramic layer in driving by
applying the driving voltage waveform to the piezoelectric deformation
region of the piezoelectric actuator is set to not more than 1.3 times of
the area of the P-E hysteresis loop of the conventional pull-push driving
voltage waveform shown in FIG. 11 and yet in the case where the driving
voltage (VH) is twice of the value of the first and second voltages
for reducing hysteresis loss. Thus, piezoelectric deformation properties
can be prevented from reduction resulting from depolarization of the
piezoelectric ceramic layer caused by self heating.

[0056]According to the driving method of the invention in claim 4, the
hysteresis loss is further reduced by setting the first and second
voltages of the driving voltage waveform to such a value that the
intensity of electric field E (kV/cm) of the piezoelectric deformation
region of the piezoelectric actuator is not more than 0.8 times of the
intensity of the coercive electric field Ec of the piezoelectric ceramic
layer. Accordingly, the piezoelectric deformation properties can be
further reliably prevented from reduction resulting from depolarization
of the piezoelectric ceramic layer caused by self heating.

[0057]According to the driving method of the invention in claim 5, creep
deformation of the inactive region of the piezoelectric ceramic layer can
be further reliably prevented by maintaining the stationary state
applying no voltage to the piezoelectric deformation region in the
standby state not ejecting ink droplets.

[0058]The driving method according to the present invention is applicable
to a liquid ejector including any one of the unimorphic (claim 6),
bimorphic (claim 7) and monomorphic (claim 8) piezoelectric actuators, as
hereinabove described. In anyone of these cases, the ink droplet ejection
performance can be maintained at an excellent level over a long period by
preventing gradual creep deformation of the inactive region surrounding
the active regions of the piezoelectric ceramic layer and preventing
destabilization of ejection of ink droplets resulting from occurrence of
noise vibration in the driving state of the piezoelectric deformation
region.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059][FIG. 1] A graph showing the relation between an example of a
driving voltage waveform of a driving voltage VP applied to an
active region of a piezoelectric ceramic layer when a liquid ejector
shown in FIG. 2 is driven by a driving method according to the present
invention and changes of the volume velocity of ink in a nozzle upon
application of this driving voltage waveform.

[0060][FIG. 2] A sectional view showing an example of a liquid ejector
including a unimorphic piezoelectric actuator employed for an on-demand
ink jet printer or the like.

[0061][FIG. 3] A sectional view showing a principal part of the example of
the liquid ejector in an enlarged manner.

[0062][FIG. 4] A graph showing the relation between examples of the
driving voltage waveform of a driving voltage VP1 applied to an
active region of a first piezoelectric ceramic layer and the driving
voltage waveform of a driving voltage VP2 applied to an active
region of a second piezoelectric ceramic layer when a liquid ejector of
an example shown in FIG. 5 is driven by the driving method according to
the present invention and changes of the volume velocity of ink in a
nozzle upon application of these driving voltage waveforms in a
simplified manner.

[0063][FIG. 5] A sectional view showing the example of the liquid ejector
including a bimorphic piezoelectric actuator.

[0064][FIG. 6] A sectional view showing an example of a liquid ejector
including a monomorphic piezoelectric actuator.

[0065][FIG. 7] A graph showing results of measurement of driving lives in
driving of a liquid ejector including a unimorphic piezoelectric actuator
manufactured according to Example 1 of the present invention by the
driving method according to the present invention and a conventional
pull-push driving method.

[0066][FIG. 8] A graph showing the relation between displacements of a
piezoelectric deformation region of the piezoelectric actuator in the
thickness direction and applied voltages in driving of the liquid ejector
manufactured according to the aforementioned Example 1 by the driving
method according to the present invention and the conventional pull-push
driving method.

[0067][FIG. 9] A graph showing P-E hysteresis characteristics measured at
various voltages applied in the driving method according to the present
invention as to the piezoelectric ceramic layer of the liquid ejector
manufactured according to the aforementioned Example 1.

[0068][FIG. 10] A graph showing P-E hysteresis characteristics measured by
applying voltage waveforms corresponding to the driving method according
to the present invention and the conventional pull-push driving method as
to the piezoelectric ceramic layer of the liquid ejector manufactured
according to the aforementioned Example 1.

[0069][FIG. 11] A graph showing the relation between an example of the
driving voltage waveform of the driving voltage VP applied to the
active region of the piezoelectric ceramic layer when the liquid ejector
shown in FIG. 2 is driven by the conventional pull-push driving method
and changes of the volume velocity of the ink in the nozzle upon
application of this driving voltage waveform in a simplified manner.

[0070][FIG. 12] A graph showing the relation between the example of the
driving voltage waveform of the driving voltage VP applied to the
active region of the piezoelectric ceramic layer when the liquid ejector
shown in FIG. 2 is driven by the pull-push driving method and the
displacement of the piezoelectric deformation region of the piezoelectric
actuator upon application of this driving voltage waveform in a
simplified manner.

DESCRIPTION OF THE REFERENCE NUMERALS

[0071]-VL first voltage

[0072]+VL second voltage

[0073]1 liquid ejector

[0074]2 pressuring chamber

[0075]3 nozzle

[0076]4 liquid droplet ejecting portion

[0077]5 substrate

[0078]6 (first) piezoelectric ceramic layer

[0079]7 piezoelectric actuator

[0080]8 piezoelectric deformation region

[0081]9 restricted region

[0082]12 oscillator plate

[0083]15 active region

[0084]16 inactive region

[0085]17 second piezoelectric ceramic layer

EMBODIMENTS OF THE INVENTION

[0086]FIG. 1 is a graph showing the relation between an example of a
driving voltage waveform (shown by a thick one-dot chain lines) of a
driving voltage VP applied to the active region 15 of the
piezoelectric ceramic layer 6 when the liquid ejector 1 shown in FIG. 2
is driven by the driving method according to the present invention and
changes [shown by a thick solid line, (+) denotes the distal end of the
nozzle 3, i.e., the ink droplet ejection side, and (-) denotes the side
of the pressurizing chamber 2] of the volume velocity of the ink in the
nozzle 3 upon application of this driving voltage waveform. FIG. 2 is a
sectional view showing the example of the liquid ejector 1 including the
unimorphic piezoelectric actuator 7 employed for an on-demand ink jet
printer or the like.

[0087]Referring to FIGS. 2 and 3, the liquid ejector 1 of this example
includes, as hereinabove described, a substrate 5 formed by arranging a
plurality of liquid droplet ejecting portions 4 each having a
pressurizing chamber 2 to be filled with the ink and a nozzle 3 for
ejecting the ink from the pressurizing chamber 2 as an ink droplet in the
plane direction and the plate-shaped piezoelectric actuator 7, including
a piezoelectric ceramic layer 6 having a size covering the plurality of
pressurizing chambers 2 of the substrate 5, laminated on the substrate 5.

[0088]The piezoelectric actuator 7 is divided into a plurality of
piezoelectric deformation regions 8 arranged correspondingly to the
respective pressurizing chambers 2 and individually deflected in the
thickness direction by individual voltage application and a restricted
region 9 arranged to surround the piezoelectric deformation regions 8 and
fixed to the substrate 5 to be prevented from deformation. Further, the
piezoelectric actuator 7 of the example shown in figures has a so-called
unimorphic structure including individual electrodes 10 individually
formed on the upper surface of the piezoelectric ceramic layer 6 in both
figures correspondingly to the respective pressuring chambers 2 for
defining the piezoelectric deformation regions as well as a common
electrode 11 and the oscillator plate 12 successively laminated on `the
lower surface of the piezoelectric ceramic layer 6 each having a size
covering the plurality of pressurizing chambers 2. The individual
electrodes 10 and the common electrode 11 are separately connected to the
driving circuit 13, and the driving circuit 13 is connected to the
control unit 14.

[0089]The piezoelectric ceramic layer 6 is made of a piezoelectric
material such as PZT, for example, and previously polarized in the
thickness direction to have piezoelectric deformation properties of
so-called transverse vibration mode. When the driving circuit 13 is
driven by a control signal from the control unit 14 and a voltage of the
same direction ((+) direction in FIG. 1) as the direction of the
polarization is applied between an arbitrary individual electrode 10 and
the common electrode 11, an active region 15 corresponding to the
piezoelectric deformation region 8 sandwiched between these electrodes 10
and 11 is contracted in the layer plane direction, as shown by the white
lateral arrows in FIG. 3. Thus, the piezoelectric deformation region 8 of
the piezoelectric actuator 7 is deflected so as to protrude in the
direction of the pressurizing chamber 2 as shown by the white downward
arrow in FIG. 3, since the lower surface of the piezoelectric ceramic
layer 6 is fixed to the oscillator plate 12 through the common electrode
11.

[0090]When a voltage in the direction ((-) direction in FIG. 1) opposite
to the direction of polarization is applied between the individual
electrode 10 and the common electrode 11, on the other hand, the active
region 15 is expanded in the layer plane direction oppositely to the
lateral arrows in FIG. 3, whereby the piezoelectric deformation region 8
of the piezoelectric actuator 7 is deflected in the direction opposite to
the pressurizing chamber 2, as shown by an upward arrow in FIG. 3.
Therefore, the ink filled in the pressurizing chamber 2 can be vibrated
and ejected through the nozzle 3 as ink droplets by repeating the
deflection of the piezoelectric deformation region 8 in the direction of
the pressurizing chamber 2 and the deflection in the direction opposite
thereto.

[0091]Referring to FIGS. 1 to 3, a state not applying the driving voltage
VP (VP=0) but releasing the piezoelectric deformation region 8
from deflection is maintained in a standby state on the left side of
t1 in FIG. 1 not ejecting ink droplets from the nozzle 3, while the
ink remains in a stationary state, i.e., the volume velocity of the ink
in the nozzle 3 is maintained at 0, and an ink meniscus formed in the
nozzle 3 by the surface tension of the ink remains stationary.

[0092]In order to form dots on a sheet surface by ejecting ink droplets
from the nozzle 3, the driving voltage VP is charged
(VP=-VL) to a first voltage (-VL) opposite to the
direction of polarization at the preceding time t1 for expanding the
active region 15 in the plane direction, thereby deflecting the
piezoelectric deformation region 8 in the direction opposite to the
pressurizing chamber 2. Thus, the volume of the pressurizing chamber 2 is
increased by a certain amount, whereby the ink meniscus in the nozzle 3
is drawn into the pressurizing chamber 2 by this increment of the volume.
At this time, the volume velocity of the ink in the nozzle 3 is
temporarily increased toward the (-) side and thereafter gradually
reduced to finally approach 0, as shown in the portion between t1
and t2 in FIG. 1. This corresponds to generally a half cycle of the
natural vibration cycle T1 of the volume velocity of the ink shown
by a thick solid line.

[0093]At the time t2 when the volume velocity of the ink in the nozzle 3
infinitely approaches 0, the driving voltage VP is charged
(VP=+VL) to a second voltage (+VL) of the same direction
as the direction of polarization for contracting the active region 15 in
the plane direction, thereby deflecting the piezoelectric deformation
region 8 so as to protrude in the direction of the pressurizing chamber
2.

[0094]Thus, the volume of the pressurizing chamber 2 is reduced due to the
deflection of the piezoelectric deformation region 8 in the direction of
the pressurizing chamber 2 so that the pressure of the ink extruded from
the pressurizing chamber 2 is applied to the ink in the nozzle 3 going to
return in the direction of the distal end of the nozzle 3 contrarily to
the state where the ink meniscus is most remarkably drawn into the
pressurizing chamber 2 (the state where the volume velocity is 0 at the
time t2). As a result, the ink in the nozzle 3 is accelerated in the
direction of the distal end of the nozzle 3 to remarkably protrude
outward from the nozzle 3. At this time, the volume velocity of the ink
in the nozzle 3 is temporarily increased toward the (+) side and
thereafter gradually reduced to finally approach 0, as shown in the
portion between t2 and t3 in FIG. 1. Thus, the aforementioned
ink column is formed.

[0095]At the time (t3 in FIG. 1) when the volume velocity of the ink
protruding outward from the nozzle 3 infinitely approaches 0, the driving
voltage VP is charged (VP=-VL) to the first voltage
(-VL) again for expanding the active region 15 in the plane
direction, thereby deflecting the piezoelectric deformation region 8 in
the direction opposite to the pressurizing chamber 2. Thus, a negative
pressure formed by deflecting the piezoelectric deformation region 8 in
the direction opposite to the pressurizing chamber 2 and increasing the
volume of the pressurizing chamber 2 again is applied to the ink going to
return into the pressurizing chamber 2 contrarily to the state most
remarkably protruding outward of the nozzle 3 (the state where the volume
velocity is 0 at the time t3). As a result, the ink column extending
from the nozzle 3 to the utmost is cut off to form a first ink droplet.

[0096]After the ink column is cut off, the ink in the nozzle 3 is drawn
into the pressurizing chamber 2 again. At this time, the volume velocity
of the ink in the nozzle 3 is temporarily increased toward the (-) side
and thereafter gradually reduced to finally approach 0, as shown in the
portion between t3 and T4 in FIG. 1. This corresponds to
generally a half cycle of the natural vibration cycle T1 of the
volume velocity of the ink, as hereinabove described.

[0097]At the time t4 when the volume velocity of the ink in the
nozzle 3 infinitely approaches 0, the driving voltage VP is charged
(VP=+VL) to the second voltage (+VL) again for contracting
the active region 15 in the plane direction, thereby deflecting the
piezoelectric deformation region 8 in the direction of the pressurizing
chamber 2. Thus, the ink remarkably protrudes outward from the nozzle 3
again to form an ink column, due to the same mechanism as that of the
aforementioned behavior of the ink between the times t2 and t3.
At this time, the volume velocity of the ink in the nozzle 3 is
temporarily increased toward the (+) side and thereafter gradually
reduced to finally approach 0, as shown in the portion between t4
and t5 in FIG. 1.

[0098]After the time (t5 in FIG. 1) when the volume velocity of the
ink in the nozzle 3 reaches 0, the speed of vibration of the ink is
directed toward the pressurizing chamber 2, whereby the ink column
extending from the nozzle 3 to the utmost is cut off to form a second ink
droplet. The first and second ink droplets formed in this manner spatter
onto the sheet surface opposed to the distal end of the nozzle 3
individually to form one dot.

[0099]The series of operations correspond to application of the driving
voltage VP having the driving voltage waveform including two pulses
each having a pulse width T2 of about half of the natural vibration
cycle T1 to the active region 15, as shown by the thick one-dot
chain lines in FIG. 1. In order to form one dot with only one ink
droplet, the driving voltage waveform may include only one pulse. In
order to form one dot with not less than three ink droplets, the pulse
may be generated by the frequency corresponding to the number of the ink
droplets.

[0100]In a case of subsequently forming a next dot after termination of
the series of operations, the operation starting from t1 is repeated
again. In a case of not forming the next dot, on the other hand, the
apparatus is brought into the standby state not applying (VP=0) the
driving voltage VP.

[0101]According to the driving method of this example, the inactive region
16 of the piezoelectric ceramic layer 6 corresponding to the restricted
region 9 of the unimorphic piezoelectric actuator 7 can be prevented from
gradual creep deformation by performing the series of operations.

[0102]In other words, the piezoelectric deformation region 8 of the
piezoelectric actuator 7 is deflected in the respective directions
opposite to the pressurizing chamber 2 and the direction of the
pressurizing chamber 2 by applying the driving voltage waveform including
the first voltage (-VL) and the second voltage (+VL) opposite
in polarity to the first voltage and equivalent thereto in ejection of
ink droplet. Accordingly, the active region 15 of the piezoelectric
ceramic layer 6 can be not only contracted in the plane direction and
released from the contraction similarly to the conventional piezoelectric
actuator but also expanded in the plane direction. Therefore, the
inactive region 16 surrounding the active region 15 can be prevented from
gradual creep deformation.

[0103]According to the driving method of this example, further, the
displacement of the piezoelectric deformation region 8 in the thickness
direction with respect to the stationary state of the piezoelectric
actuator 7 not subjected to voltage application can be further reduced as
compared with the prior art. In the driving method of this example,
assuming that the displacement in the thickness direction between the
stationary state (VP=0) and the deflected state (VP=VH) in
the conventional driving method shown in FIG. 11 is 1, the displacements
for deflecting the piezoelectric deformation region 8 in the direction
opposite to the pressurizing chamber 2 and the direction of the
pressurizing chamber 2 for setting the total displacement of the
piezoelectric deformation region 8 in the thickness direction identically
to 1 in the driving method of this example can be each generally halved.

[0104]Therefore, stress in the plane direction applied to the inactive
region 16 of the piezoelectric ceramic layer 6 upon deflection of the
piezoelectric deformation region 8 can be further reduced. Therefore, the
inactive region 16 can be more reliably prevented from creep deformation
in combination that the stationary state is maintained applying no
voltage to the piezoelectric deformation region 8 in the standby state
not ejecting ink droplets.

[0105]In the driving method of this example, further, the displacement of
the deflection of the piezoelectric deformation region 8 in the standby
state can be generally halved as compared with the conventional one as
hereinabove described. As a result, storage of elastic energy in the
piezoelectric deformation region 8 in the standby state can be reduced
and the shape of the piezoelectric deformation region 8 can be
constrained by voltage application in both of the standby state and the
driving state, thereby suppressing occurrence of noise vibration.
Therefore, destabilization of ejection of ink droplets from the nozzle 3
corresponding to the piezoelectric deformation region 8 as well as
destabilization of ejection of ink droplets from the nozzle 3
corresponding to the adjacent piezoelectric deformation region 8
resulting from occurrence of a crosstalk can be prevented.

[0106]According to the driving method of this example, therefore, the ink
droplet ejection performance can be maintained at an excellent level over
a long period by preventing gradual creep deformation of the inactive
region 16 of the piezoelectric ceramic layer 6 corresponding to the
restricted region 9 of the unimorphic piezoelectric actuator 7 and
preventing destabilization of ejection of ink droplets resulting from
noise vibration caused in the driving state of the piezoelectric
deformation region 8.

[0107]According to the driving method of this example, further, the
inactive region 16 of the piezoelectric ceramic layer 6 can be prevented
from creep deformation as hereinabove described. As a result, the
crystalline state of the inactive region 16 can be prevented from
changing, and the crystalline state of the active region 15 can also be
prevented from changing due to compressive stress received from the
creep-deformed inactive region 16. Therefore, the crystalline states of
both regions 15 and 16 of the piezoelectric ceramic layer 6 can be
maintained in the initial states.

[0108]When the piezoelectric ceramic layer 6 is made of a PZT-type
piezoelectric ceramic material, for example, both of the active region 15
and the inactive region 16 can be maintained so that the C-axis
orientation IC showing the crystalline state of the ceramic material
obtained from the intensity I.sub.(200) of the diffraction peak of the
[200] plane and the intensity I.sub.(002) of the diffraction peak of the
[002] plane in an X-ray diffraction spectrum by the following expression
(1):

IC=I.sub.(002)/I.sub.(002)+I.sub.(200)) (1)

is kept in the range of 1 to 1.1 times as that in the undriven initial
state after driving.

[0109]In the driving method of this example as hereinabove described, when
the displacements of the piezoelectric deformation region 8 in the
direction opposite to the pressurizing chamber 2 and the direction of the
pressurizing chamber 2 are each set to about half of the displacement in
one direction in the conventional driving method, the absolute value of
the first and second voltages -VL and +VL applied to the active
region 15 of the piezoelectric ceramic layer 6 can also be set to about
half of the absolute value of the driving voltage VH in the
conventional driving method. Therefore, the insulating structure or the
like can also be advantageously simplified by reducing the withstanding
voltage value of the circuit reaching the electrodes 10 and 11 from the
driving circuit 13. This is because the displacement of the deflection of
the piezoelectric deformation region 8 in the thickness direction is
proportionate to the value of the driving voltage applied to the active
region 15 of the piezoelectric ceramic layer 6 in the unimorphic
piezoelectric actuator 7 including the piezoelectric ceramic layer 6
imparted with the piezoelectric deformation properties of the transverse
vibration mode in general.

[0110]The area of the P-E hysteresis loop showing the relation between the
intensity of electric field E (kV/cm),and the polarization quantity P
(μC/cm2) of the piezoelectric ceramic layer 6 at the time of
applying the driving voltage waveform to the piezoelectric deformation
region 8 of the piezoelectric actuator 7 and driving the same is
preferably set to not more than 1.3 times of the area of the P-E
hysteresis loop of the conventional pull-push driving voltage wave shown
in FIG. 11 and yet in the case where the driving voltage VH is twice
of the value of the first voltage (-VL) and the second voltage
(+VL). Thus, the hysteresis loss is so reduced that the
piezoelectric deformation properties can be prevented from reduction
resulting from depolarization of the piezoelectric ceramic layer 6 caused
by self heating. Therefore, the ink droplet ejection performance can be
maintained at an excellent level over a longer period.

[0111]In consideration of minimization of the hysteresis loss, the area of
the P-E hysteresis loop is preferably set to not less than one time, more
preferably 1.01 to 1.20 times of the area of the P-E hysteresis loop in
the case of the conventional pull-push method. In order to adjust the
area of the P-E hysteresis loop in the aforementioned range, the values
of the first voltage (-VL) and the second voltage (+VL) are
preferably minimized. More specifically, the area of the P-E hysteresis
loop is abruptly increased when the first and second voltages are set to
such values that the intensity of electric field E of the piezoelectric
deformation region 8 of the piezoelectric actuator 7 exceeds the
intensity of the coercive electric field Ec of the piezoelectric ceramic
layer 6. Accordingly, the first and second voltages are preferably set to
such values that the intensity of electric field E of the piezoelectric
deformation region 8 of the piezoelectric actuator 7 is not more than the
intensity of the coercive electric field Ec of the piezoelectric ceramic
layer 6.

[0112]It is also effective to apply compressive stress to the entire
piezoelectric ceramic layer 6 in order to adjust the area of the P-E
hysteresis loop in the aforementioned range. In other words, polarization
inversion is hardly caused when compressive stress is applied to the
entire piezoelectric ceramic layer 6. Therefore, the area of the P-E
hysteresis loop can be reduced by increasing the compressive stress if
the electric field remains the same.

[0113]When the first and second voltages -VL and +VL, are set to
such values that the intensity of electric field E of the piezoelectric
deformation region 8 of the piezoelectric actuator 7 is not more than 0.8
times, particularly 0.5 to 0.7 times of the intensity of the coercive
electric field Ec of the piezoelectric ceramic layer 6, the
aforementioned effect of preventing depolarization to prevent reduction
of the piezoelectric deformation properties can be rendered more
reliable. Therefore, the ink droplet ejection performance can be
maintained at an excellent level over a longer period.

[0114]FIG. 5 is a sectional view showing an example of a liquid ejector 1
including a bimorphic piezoelectric actuator 7. Referring to FIG. 5, the
liquid ejector 1 of this example is identical in structure to the
aforementioned liquid ejector 1 shown in FIG. 2 except the piezoelectric
actuator 7. Therefore, identical portions are denoted by the same
reference numerals, and description is omitted. The piezoelectric
actuator 7 is divided into a plurality of piezoelectric deformation
regions 8 arranged correspondingly to respective pressurizing chambers 2
and individually deflected in the thickness direction by individual
voltage application and a restricted region 9 arranged to surround the
piezoelectric deformation regions 8 and fixed to the substrate 5 to be
prevented from deformation.

[0115]The piezoelectric actuator 7 includes a first piezoelectric ceramic
layer 6 having a size covering the plurality of pressurizing chambers 2
arranged on the substrate 5 and individual electrodes 10 individually
formed on the upper surface of the first piezoelectric ceramic layer 6
correspondingly to the respective pressurizing chambers 2 for defining
the piezoelectric deformation regions 8, as well as a first common
electrode 11, a second piezoelectric ceramic layer 17 and a second common
electrode 18 successively laminated on the lower surface of the first
piezoelectric ceramic layer 6 each having a size covering the plurality
of pressurizing chambers 2, and has the bimorphic structure, as
hereinabove described. The individual electrodes 10 and the first and
second common electrodes 11 and 18 are separately connected to a driving
circuit 13, and the driving circuit 13 is connected to control unit 14.

[0116]The first piezoelectric ceramic layer 6 is made of a piezoelectric
material such as PZT, for example, and previously polarized in the layer
thickness direction to have piezoelectric deformation properties of the
transverse vibration mode. When the driving circuit 13 is driven by a
control signal from the control unit 14 and a voltage of the same
direction as the direction of the polarization is applied between an
arbitrary individual electrode 10 and the first common electrode 11, an
active region 15 corresponding to the piezoelectric deformation region 8
sandwiched between these electrodes 10 and 11 is contracted in the layer
plane direction. When a voltage opposite to the direction of polarization
is applied between the electrodes 10 and 11, on the other hand, the
active region 15 is contrarily expanded in the layer plane direction.

[0117]On the other hand, the second piezoelectric ceramic layer 17 is
similarly made of a piezoelectric material such as PZT, and previously
polarized in the layer thickness direction to have piezoelectric
deformation properties of the transverse vibration mode. Further, the
second piezoelectric ceramic layer 17 is divided into active regions 19
corresponding to the piezoelectric deformation regions 8 contracted in
the layer plane direction when the driving circuit 13 is driven by the
control signal from the control unit 14 and the voltage of the same
direction as the direction of the polarization is applied between the
first and second common electrodes 11 and 18 and expanded in the layer
plane direction when the voltage of the opposite direction is applied and
an inactive region 20 fixed to the substrate 5 and restricted in
expansion/contraction despite voltage application from the common
electrodes 11 and 18.

[0118]When the voltage opposite to the direction of polarization is
applied to the entire second piezoelectric ceramic layer 17 for expanding
the active regions 19 in the plane direction synchronously with
application of the voltage of the same direction as the direction of the
polarization between an arbitrary individual electrode 10 of the first
piezoelectric ceramic layer 6 and the first common electrode 11 for
contracting the corresponding active region 15 in the plane direction in
the bimorphic piezoelectric actuator 7, the piezoelectric deformation
region 8 of the piezoelectric actuator 7 is deflected so as to protrude
in the direction of the pressurizing chamber 2 accordingly.

[0119]When the voltage of the same direction as the direction of
polarization is applied to the entire second piezoelectric ceramic layer
17 for contracting the active regions 19 in the plane direction
synchronously with application of the voltage opposite to the direction
of polarization between an arbitrary individual electrode 10 of the first
piezoelectric ceramic layer 6 and the first common electrode 11 for
expanding the corresponding active region 15 in the plane direction, on
the other hand, the piezoelectric deformation region 8 of the
piezoelectric actuator 7 is deflected so as to protrude oppositely to the
pressurizing chamber 2 accordingly. Therefore, ink filled in the
pressurizing chamber 2 can be vibrated and ejected through the nozzle 3
as ink droplets by repeating the deflection of the piezoelectric
deformation region 8 in the direction of the pressurizing chamber 2 and
in the direction opposite thereto.

[0120]FIG. 4 is a graph showing the relation between examples of the
driving voltage waveform (shown by thick one-dot chain lines in the upper
stage of FIG. 4) of a driving voltage VP1 applied to the active
region 15 of the first piezoelectric ceramic layer 6 and the driving
voltage waveform (shown by thick one-dot chain lines in the lower stage
of FIG. 4) of a driving voltage VP2 applied to the second
piezoelectric ceramic layer 17 when the liquid ejector 1 of the example
shown in FIG. 5 is driven by the driving method according to the present
invention and changes of the volume velocity of the ink in the nozzle 3
upon application of these driving voltage waveforms in a simplified
manner.

[0121]Referring to FIGS. 4 and 5, in the standby state on the left side of
t1 in FIG. 4 not ejecting ink droplets from the nozzle 3, a state
not applying the driving voltages Vp1 and VP2 (VP1=0,
VP2=0) and releasing the piezoelectric deformation region 8 from
deflection is maintained while the ink remains in a stationary state,
i.e., the volume velocity of the ink in the nozzle 3 is maintained at 0,
and an ink meniscus formed in the nozzle 3 by the surface tension of the
ink remains stationary.

[0122]In order to eject ink droplets from the nozzle 3 and form dots on a
sheet surface, the driving voltage VP1 is charged
(VP1=-VL1) to a first voltage (-VL1) opposite to the
direction of polarization at the preceding time t1 for expanding the
active region 15 in the plane direction while the driving voltage
VP2 is charged (VP2=+VL2) to a first voltage (+VL2)
of the same direction as the direction of polarization for contracting
the active region 19 in the plane direction, thereby deflecting the
piezoelectric deformation region 8 in the direction opposite to the
pressurizing chamber 2.

[0123]Thus, the volume of the pressurizing chamber 2 is increased by a
certain amount, whereby the ink meniscus in the nozzle 3 is drawn into
the pressurizing chamber 2 by this increment of the volume. At this time,
the volume velocity of the ink in the nozzle 3 is temporarily increased
toward the (-) side and thereafter gradually reduced to finally approach
0, as shown in the portion between t1 and t2 in FIG. 4.

[0124]At the time t2 when the volume velocity of the ink in the
nozzle 3 infinitely approaches 0, the driving voltage VP1 is charged
(VP1=+VL1) to a second voltage (+VL1) of the same
direction as the direction of polarization for contracting the active
region 15 in the plane direction while the driving voltage VP2 is
charged (VP2=-VL2) to a second voltage (-VL2) opposite to
the direction of polarization for expanding the active region 19 in the
plane direction, thereby deflecting the piezoelectric deformation region
8 to protrude in the direction of the pressurizing chamber 2.

[0125]Thus, the volume of the pressurizing chamber 2 is reduced due to the
deflection of the piezoelectric deformation region 8 in the direction of
the pressurizing chamber 2 so that the pressure of the ink extruded from
the pressurizing chamber 2 is applied to the ink in the nozzle 3 going to
return in the direction of the distal end of the nozzle 3 contrarily to
the state where the ink meniscus is most remarkably drawn into the
pressurizing chamber 2 (the state where the volume velocity is 0 at the
time t2). Thus, the ink in the nozzle 3 is accelerated in the
direction of the distal end of the nozzle 3 to remarkably protrude
outward from the nozzle 3. At this time, the volume velocity of the ink
in the nozzle 3 is temporarily increased toward the (+) side and
thereafter gradually reduced to finally approach 0, as shown in the
portion between t2 and t3 in FIG. 4. Thus, the aforementioned
ink column is formed.

[0126]At the time (t3 in FIG. 4) when the volume velocity of the ink
protruding outward from the nozzle 3 infinitely approaches 0, the driving
voltage VP1 is charged (VP1=-VL1) to the first voltage
(-VL1) again for expanding the active region 15 in the plane
direction while the driving voltage VP2 is charged
(VP2=+VL2) to the first voltage (+VL2) again for
contracting the active region 19 in the plane direction, thereby
deflecting the piezoelectric deformation region 8 in the direction
opposite to the pressurizing chamber 2.

[0127]Thus, a negative pressure formed by deflecting the piezoelectric
deformation region 8 in the direction opposite to the pressurizing
chamber 2 and increasing the volume of the pressurizing chamber 2 again
is applied to the ink going to return into the pressurizing chamber 2
contrarily to the state most remarkably protruding outward of the nozzle
3 (the state where the volume velocity is 0 at the time t3), whereby
the ink column extending from the nozzle 3 to the utmost is cut off to
form a first ink droplet. After the ink column is cut off, the ink in the
nozzle 3 is drawn into the pressurizing chamber 2 again. At this time,
the volume velocity of the ink in the nozzle 3 is temporarily increased
toward the (-) side and thereafter gradually reduced to finally approach
0, as shown in the portion between t3 and T4 in FIG. 4.

[0128]At the time t4 when the volume velocity of the ink in the
nozzle 3 infinitely approaches 0, the driving voltage VP1 is charged
(VP1=+VL1) to the second voltage (+VL1) again for
contracting the active region 15 in the plane direction while the driving
voltage VP2 is charged (VP2=-VL2) to the second voltage
(-VL2) again for expanding the active region 19 in the plane
direction, thereby deflecting the piezoelectric deformation region 8 in
the direction of the pressurizing chamber 2. Thus, the ink remarkably
protrudes outward from the nozzle 3 again to form an ink column, due to
the same mechanism as that of the aforementioned behavior of the ink
between the times t2 and t3. At this time, the volume velocity
of the ink in the nozzle 3 is temporarily increased toward the (+) side
and thereafter gradually reduced to finally approach 0, as shown in the
portion between t4 and t5 in FIG. 4.

[0129]After the time (t5 in FIG. 4) when the volume velocity of the
ink in the nozzle 3 reaches 0, the speed of vibration of the ink is
directed toward the pressurizing chamber 2, whereby the ink column
extending from the nozzle 3 to the utmost is cut off to form a second ink
droplet. The first and second ink droplets formed in this manner spatter
onto the sheet surface opposed to the distal end of the nozzle 3
individually, to form one dot.

[0130]The series of operations correspond to application of the driving
voltage VP1 having the driving voltage waveform including two pulses
each having a pulse width T2 of about half of the natural vibration
cycle T1 to the active region 15 while applying the driving voltage
VP2 having an antiphase driving voltage waveform synchronous
therewith to the second piezoelectric ceramic layer 17, as shown by thick
one-dot chain lines in FIG. 4. In order to form one dot with only one ink
droplet, the driving voltage waveform may include only one pulse. In
order to form one dot with not less than three ink droplets, the pulse
may be generated by the frequency corresponding to the number of the ink
droplets. In a case of subsequently forming a next dot after termination
of the series of operations, the operation starting from t1 is
repeated again. In a case of not forming the next dot, on the other hand,
the apparatus is brought into the standby state not applying both of the
driving voltages Vp1 and VP2 (VP1=0, VP2=0).

[0131]According to the driving method of this example, the inactive region
16 of the first piezoelectric ceramic layer 6 and the inactive region 20
of the second piezoelectric ceramic layer 17 corresponding to the
restricted region 9 of the bimorphic piezoelectric actuator 7 each can be
prevented from gradual deformation by performing the series of
operations.

[0132]Similarly to the aforementioned case of the unimorphic piezoelectric
actuator 7, plane-directional stress applied to both inactive regions 16
and 20 upon deflection of the piezoelectric deformation region 8 can be
reduced as compared with the conventional one by setting the
displacements for deflecting the piezoelectric deformation region 8 in
the direction opposite to the pressurizing chamber 2 and the direction of
the pressurizing chamber 2 with respect to the stationary state applying
no voltage to about half of that in the conventional method for driving
the bimorphic piezoelectric actuator 7. As a result, the inactive regions
16 and 20 can be more reliably prevented from creep deformation in
combination that the stationary state is maintained applying no voltage
to the piezoelectric deformation region 8 in the standby state not
ejecting ink droplets.

[0133]Further, the displacement of the deflection of the piezoelectric
deformation region 8 in the standby state can be generally halved as
compared with the conventional one. As a result, storage of elastic
energy in the piezoelectric deformation region 8 in the standby state can
be reduced and the shape of the piezoelectric deformation region 8 can be
constrained by voltage application in both of the standby state and the
driving state, thereby suppressing occurrence of noise vibration.
Therefore, destabilization of ejection of ink droplets from the nozzle 3
corresponding to the piezoelectric deformation region 8 as well as
destabilization of ejection of ink droplets from the nozzle 3
corresponding to the adjacent piezoelectric deformation region 8
resulting from occurrence of a crosstalk can be prevented.

[0134]According to the driving method of this example, therefore, the ink
droplet ejection performance can be maintained at an excellent level over
a long period by preventing gradual creep deformation of the inactive
region 16 of the first piezoelectric ceramic layer 6 and the inactive
region 20 of the second piezoelectric ceramic layer 17 corresponding
to,the restricted region 9 of the bimorphic piezoelectric actuator 7 and
preventing destabilization of ejection of ink droplets resulting from
noise vibration caused in the driving state of the piezoelectric
deformation region 8.

[0135]When both of the first and second piezoelectric ceramic layers 6 and
17 are made of a PZT-type piezoelectric ceramic material, for example,
the crystalline states of all of the active regions 15 and 19 and the
inactive regions 16 and 20 can be maintained so that the C-axis
orientation IC showing the crystalline state of the ceramic material
obtained from the intensity I.sub.(200) of the diffraction peak of the
[200] plane and the intensity I.sub.(002) of the diffraction peak of the
[002] plane in the X-ray diffraction spectrum by the following expression
(1):

IC=I.sub.(002)/(I.sub.(002)+I.sub.(200)) (1)

is kept in the range of 1 to 1.1 times as that in the undriven initial
state after driving according to the driving method of this example.

[0136]When the displacements of the piezoelectric deformation region 8 in
the direction opposite to the pressurizing chamber 2 and the direction of
the pressurizing chamber 2 are each set to about half of the displacement
in one direction in the conventional driving method, the absolute values
of the first and second voltages -VL1 and +VL1 applied to the
active region 15 of the first piezoelectric ceramic layer 6 and the
absolute values of the first and second voltages +VL2 and -VL2
applied to the second piezoelectric ceramic layer 17 can be set to about
half of the driving voltage value in the conventional driving method.
Therefore, the insulating structure or the like can also be
advantageously simplified by reducing the withstanding voltage value of
the circuit reaching the electrodes 10 and 11 from the driving circuit
13. The reason for this is similar to that in the case of the
aforementioned unimorphic piezoelectric actuator 7. In other words, the
displacement of the deflection of the piezoelectric deformation region 8
in the thickness direction is proportionate to the values of the driving
voltages applied to the active region 15 of the first piezoelectric
ceramic layer 6 and the second piezoelectric ceramic layer 17.

[0137]In the bimorphic piezoelectric actuator 7, the values of the
respective driving voltages applied to the first and second piezoelectric
ceramic layers 6 and 17 can be set to about half of the value of the
driving voltage applied to the piezoelectric ceramic layer of the
unimorphic piezoelectric actuator having the piezoelectric deformation
region set to the same displacement. According to the driving method of
this example, therefore, the absolute values of the respective voltages
-VL1, +VL1, +VL2 and -VL2 can be set to about 1/4 of
each driving voltage value VH in the conventional driving method for
the unimorphic piezoelectric actuator shown in FIG. 11.

[0138]Further, the first and second piezoelectric ceramic layers 6 and 17
can be prevented from depolarization for preventing reduction of the
piezoelectric deformation properties by setting the area of the P-E
hysteresis loop showing the relation between the intensity of electric
field E (kV/cm) and the polarization quantity P (μC/cm2) of the
piezoelectric ceramic layer at the time of applying the driving voltage
waveform to the piezoelectric deformation region 8 of the piezoelectric
actuator 7 for driving the same to not more than 1.3 times of the area of
the P-E hysteresis loop of the conventional pull-push driving voltage
waveform (applied to the first piezoelectric ceramic layer 6) shown in
FIG. 11 and an antiphase driving voltage waveform (applied to the second
piezoelectric ceramic layer 17, not shown) in the case where the driving
voltage VH is twice of the values of the respective voltages
-VL1, +VL1, -VL2 and +VL2.

[0139]In consideration of minimization of the hysteresis loss, the area of
the P-E hysteresis loop is preferably set to not less than one time, more
preferably 1.01 to 1.20 times of the area of the P-E hysteresis loop in
the case of the conventional pull-push method. In order to adjust the
area of the P-E hysteresis loop in the aforementioned range, the
respective voltages -VL1, +VL1, -VL2 and +VL2 are
preferably set to such values that the intensity of electric field E of
the piezoelectric deformation region 8 of the piezoelectric actuator 7 is
smaller than the intensity of the coercive electric field Ec of the
piezoelectric ceramic layer 6, more preferably not more than 0.8 times,
particularly preferably 0.5 to 0.7 times of the intensity of the coercive
electric field of the piezoelectric ceramic layer 6.

[0140]FIG. 6 is a sectional view showing an example of a liquid ejector 1
including a monomorphic piezoelectric actuator 7. Referring to FIG. 6,
the liquid ejector 1 of this example is identical in structure to the
aforementioned liquid ejector 1 shown in FIG. 2 except the piezoelectric
actuator 7. Therefore, identical portions are denoted by the same
reference numerals, and description is omitted. The piezoelectric
actuator 7 is divided into a plurality of piezoelectric deformation
regions 8 arranged correspondingly to respective pressurizing chambers 2
and individually deflected in the thickness direction by individual
voltage application and a restricted region 9 arranged to surround the
piezoelectric deformation regions 8 and fixed to the substrate 5 to be
prevented from deformation.

[0141]The piezoelectric actuator 7 includes a piezoelectric ceramic layer
6 having a size covering the plurality of pressurizing chambers 2
arranged on the substrate 5, individual electrodes 10 individually formed
on the upper surface of the piezoelectric ceramic layer 6 correspondingly
to the respective pressurizing chambers 2 for defining the piezoelectric
deformation regions 8 and a common electrode 11 having a size covering
the plurality of pressurizing chambers 2 formed on the lower surface of
the piezoelectric ceramic layer 6, and has a monomorphic structure, as
hereinabove described.

[0142]In other words, the piezoelectric actuator 7 is so formed that each
piezoelectric deformation region 8 can be deflected in both of the
direction opposite to the pressurizing chamber 2 and the direction of the
pressurizing chamber 2 in accordance with the direction of a voltage
applied to the piezoelectric ceramic layer 6 through the electrodes 10
and without laminating a oscillator plate or a second piezoelectric
ceramic layer by preparing the piezoelectric ceramic layer 6 from a
gradient function material or by utilizing a semiconductor effect. In the
monomorphic piezoelectric actuator 7, the piezoelectric deformation
region 8 can be vibrated similarly to that of the unimorphic
piezoelectric actuator 7 shown in FIG. 2 by applying the driving voltage
VP having the driving voltage waveform shown in FIG. 1 when the
gradient direction of the function material is selected, for example.

[0143]In other words, the driving voltage VP is not applied
(VP=0) but the piezoelectric deformation region 8 is kept released
from deflection in the standby state on the left side of t1 in FIG.
1, the driving voltage VP is charged (VP=-VL) to the first
voltage (-VL) at the time t1 for deflecting the piezoelectric
deformation region 8 in the direction opposite to the pressurizing
chamber 2 to start vibration of ink in the pressurizing chamber 2. Thus,
the driving voltage VP is charged (VP=+VL) to the second
voltage (+VL) at the time t2 for deflecting the piezoelectric
deformation region 8 to protrude in the direction of the pressurizing
chamber 2 thereby forming an ink column. The driving voltage VP is
thereafter charged (VP=-VL) to the first voltage (-VL)
again at the time t3 for deflecting the piezoelectric deformation
region 8 in the direction opposite to the pressurizing chamber 2, whereby
the ink column extending from a nozzle 3 to the utmost is cut off to form
a first ink droplet.

[0144]When the driving voltage VP is charged (VP=+VL) to
the second voltage (+VL) again at the time t4 for deflecting
the piezoelectric deformation region 8 in the direction of the
pressurizing chamber 2 and forming another ink column, the speed of
vibration of the ink is directed toward the pressurizing chamber 2 after
the time t5, whereby the ink column extending from the nozzle 3 to
the utmost is cut off to form a second ink droplet. The first and second
ink droplets formed in this manner spatter onto a sheet surface opposed
to the distal end of the nozzle 3 individually to form one dot.

[0145]The series of operations correspond to application of the driving
voltage VP having the driving voltage waveform including two pulses
each having a pulse width T2 of about half of the natural vibration
cycle T1 to the active region 15, as shown by the thick one-dot
chain lines in FIG. 1. In order to form one dot with only one ink
droplet, the driving voltage waveform may include only one pulse. In
order to form one dot with not less than three ink droplets, the pulse
may be generated by the frequency corresponding to the number of the ink
droplets. In a case of subsequently forming a next dot after termination
of the series of operations, the operation starting from t1 is
repeated again. In a case of not forming the next dot, on the other hand,
the apparatus is brought into the standby state not applying (VP=0)
the driving voltage VP.

[0146]According to the driving method of this example, performing the
series of operations can maintain the ink droplet ejection performance at
an excellent level by preventing the inactive region 16 of the
piezoelectric ceramic layer 6 corresponding to the restricted region 9 of
the monomorphic piezoelectric actuator 7 from such gradual creep
deformation that the area of the inactive region 16 in the thickness
direction corresponding to the protruding side of the active region 15 is
compressed in the plane direction and the opposite area is expanded in
the plane direction.

[0147]Similarly to the aforementioned cases of the unimorphic and
bimorphic piezoelectric actuators, plane-directional stress applied to
each inactive region 16 upon deflection of the piezoelectric deformation
region 8 can be reduced as compared with the prior art by setting the
displacements for deflecting the piezoelectric deformation region 8 in
the direction opposite to the pressurizing chamber 2 and the direction of
the pressurizing chamber 2 with respect to the stationary state applying
no voltage to about half as that in the conventional method for driving
the monomorphic piezoelectric actuator 7. As a result, each inactive
region 16 can be more reliably prevented from creep deformation in
combination that the stationary state is maintained applying no voltage
to the piezoelectric deformation region 8 in the standby state not
ejecting ink droplets.

[0148]Further, the displacement of the deflection of the piezoelectric
deformation region 8 in the standby state can be generally halved as
compared with the conventional one. As a result, storage of elastic
energy in the piezoelectric deformation region 8 in the standby state can
be reduced and the shape of the piezoelectric deformation region 8 can be
constrained by voltage application in both of the standby state and the
driving state, thereby suppressing occurrence of noise vibration.
Therefore, destabilization of ejection of ink droplets from the nozzle 3
corresponding to the piezoelectric deformation region 8 as well as
destabilization of ejection of ink droplets from the nozzle 3
corresponding to the adjacent piezoelectric deformation region 8
resulting from occurrence of a crosstalk can be prevented.

[0149]According to the driving method of this example, therefore, the ink
droplet ejection performance can be maintained at an excellent level over
a long period by preventing gradual creep deformation of each inactive
region 16 of the piezoelectric ceramic layer 6 corresponding to the
restricted region 9 of the monomorphic piezoelectric actuator 7 and
preventing destabilization of ejection of ink droplets resulting from
noise vibration caused in the driving state of the piezoelectric
deformation region 8.

[0150]When the piezoelectric ceramic layer 6 is made of a HT-type
piezoelectric ceramic material, for example, the crystalline states of
both of the active region 15 and the inactive region 16 can be maintained
so that the C-axis orientation IC showing the crystalline state of
the ceramic material obtained from the intensity I.sub.(200) of the
diffraction peak of the [200] plane and the intensity I.sub.(002) of the
diffraction peak of the [002] plane in the X-ray diffraction spectrum by
the following expression (1):

IC=I.sub.(002)/I.sub.(002)+I.sub.(200)) (1)

is kept in the range of 1 to 1.1 times as that in the undriven initial
state after driving according to the driving method of this example.

[0151]When the displacements of the piezoelectric deformation region 8 in
the direction opposite to the pressurizing chamber 2 and the direction of
the pressurizing chamber 2 are each set to about half of the displacement
in one direction in the conventional driving method, the absolute values
of the first and second voltages -VL and +VL applied to the
active region 15 of the piezoelectric ceramic layer 6 can be set to about
half of the driving voltage value in the conventional method for driving
the monomorphic piezoelectric actuator 7. Therefore, the insulating
structure or the like can also be advantageously simplified by reducing
the withstanding voltage value of the circuit reaching the electrodes 10
and 11 from the driving circuit 13.

[0152]The structure of the present invention is not limited to the example
shown in each drawing described above. Referring to the unimorphic
piezoelectric actuator 7 shown in FIG. 2, for example, the driving
voltage waveform applied to the active region 15 of the piezoelectric
ceramic layer 6 may be formed by simply changing the voltage VH in
the conventional pull-push driving method to the second voltage +VL
and changing 0 V to the first voltage -VL.

[0153]In this case, the active region 15 of the piezoelectric ceramic
layer 6 is so continuously contracted by application of the second
voltage +VL that the inactive region 16 around the same is
creep-deformed to be expanded in the plane direction in the standby
state, while this creep deformation of the inactive region 16 can be
canceled by applying the first voltage -VL in ejection of ink
droplets for forcibly expanding the active region 15. When the absolute
value of the second voltage +VL is set to about half of the voltage
VH, the amount of the creep deformation itself can be reduced.

[0154]In addition, occurrence of noise vibration can be suppressed by
reducing the displacement of the deflection of the piezoelectric
deformation region 8 as compared with the conventional one for reducing
storage of elastic energy in the piezoelectric deformation region 8 in
the standby state while constraining the shape of the piezoelectric
deformation region 8 in both in the standby state and the driving state.
Therefore, the ink droplet ejection performance can be maintained at an
excellent level over a long period by preventing the inactive region of
the piezoelectric ceramic layer surrounding the active regions from
gradual creep deformation and preventing destabilization of ejection of
ink droplets resulting from noise vibration caused in the driving state
of the piezoelectric deformation region. Further, various modifications
can be introduced in the range not departing from the subject matter of
the present invention.

Examples

Example 1

(Preparation of Piezoelectric Actuator)

[0155]Slurry was prepared by blending piezoelectric ceramic powder mainly
composed of lead zirconate titanate having a particle diameter of 0.5 to
3.0 μm with an acrylic resin emulsion and pure water and mixing these
materials with nylon balls having an average particle diameter of 10 mm
in a ball mill for 30 hours. Then, the slurry was employed for forming a
green sheet having a thickness of 17 to 19 μm for forming a
piezoelectric ceramic layer 6 and a oscillator plate 12 on a polyethylene
terephthalate (PET) film having a thickness of 30 μm by the pull-up
method.

[0156]Then, the green sheet was cut into two squares of 50 mm by 50 mm
along with the PET film was prepared, metal paste for forming a common
electrode 11 was screen-printed generally on the entire exposed surface
of one of the green sheet, and the two green sheets were thereafter dried
in an explosion-proof drier at 50° C. for 20 minutes. As the metal
paste, a powder was prepared by mixing silver powder and palladium powder
both having average particle diameters of 2 to 4 μm with each other at
a weight ratio of 7:3. A through-hole for wiring to the common electrode
11 was formed in the other green sheet.

[0157]Then, the other green sheet was overlapped on the surface printed
with the metal paste of the dried one green sheet in an aligned manner,
and held at 60° C. for 60 seconds while applying a pressure of 5
MPa in the thickness direction for thermocompression-bonding the same to
each other. Subsequently, the PET film was stripped off from both the
green sheets and filling the metal paste identical to the above into the
through-hole to form a laminate.

[0158]Then, the resin was removed from the laminate in a drier by
increasing the temperature from 100° C. to 300° C. for 25
hours at a temperature rise speed of 8° C. per hour, and
thereafter cooled to the room temperature. The laminate was further fired
in a firing furnace at a peak temperature of 1100° C. for 2 hours,
thereby obtaining a laminate of the piezoelectric ceramic layer 6, the
common electrode 11 and the oscillator plate 12. Both of the thicknesses
of the piezoelectric ceramic layer 6 and the oscillator plate 12 were 10
μm. The intensity of the coercive electric field of the piezoelectric
ceramic layer 6 was 17 kV/cm.

[0159]Then, patterns corresponding to a plurality of individual electrodes
10 were printed on the exposed surface of the piezoelectric ceramic layer
6 of the laminate by screen printing using the metal paste identical to
the above for forming the plurality of individual electrodes 10 by
passing the laminate through a continuous furnace at a peak temperature
of 850° C. for 30 minutes to bake the metal paste. The periphery
of the laminate was thereafter cut with a dicing saw to have a
rectangular contour of 33 mm by 12 mm. As patterns of individual
electrode layers 25, two rows of 90 individual electrode layers 25 were
arranged at a pitch of 254 μm along the longitudinal direction of the
rectangle to form a unimorphic piezoelectric actuator 7.

(Preparation of Liquid Ejector)

[0160]A stainless steel foil having a thickness of 100 μm was punched
with a mold press to form a first substrate having two rows each of 90
pressurizing chambers 2 of 2 mm by 0.18 mm arranged in correspondence
with a forming pitch of the individual electrodes 10. Stainless steel
foil having a thickness of 100 μm was likewise punched with a mold
press to form a second substrate having a common supply path for
supplying ink to the pressurizing chambers 2 from an ink supply section
of an ink jet printer and passages connecting the pressurizing chambers 2
and nozzles 3 arranged correspondingly to the alignment of the
pressurizing chambers 2. Still, stainless steel foil having a thickness
of 40 μm was etched to form a third substrate having nozzles 3 having
a diameter of 26 μm arranged correspondingly to the alignment of the
pressurizing chambers 2.

[0161]The first to third substrates were bonded to one another using an
adhesive to form a substrate 5. This substrate 5 and the previously
prepared piezoelectric actuator 7 were bonded to each other using an
adhesive. The respective individual electrodes 10 and exposed portions of
an electrode layer agent filled in through-holes and connected to the
common electrode 11 were connected to a driving circuit 13 with a
flexible substrate to produce the liquid ejector 1 shown in FIG. 1.

(Durability Test)

[0162]Transition of displacements of a piezoelectric deformation region 8
of the piezoelectric actuator 7 was measured when the liquid ejector 1
produced in Example 1 was continuously driven by the driving method of
the present invention and the conventional pull-push driving method using
driving voltage waveforms generated by a high-speed bipolar power source
and a function synthesizer.

[0163]In other words, the driving was stopped every certain driving cycle
(a series of operations necessary for forming one dot on a sheet surface
is assumed as one cycle) in the continuous driving. With vibrating the
piezoelectric deformation region 8 by applying a sine wave having a
frequency of 12 kHz, a vibration speed measured by applying a laser beam
to the plane of vibration thereof using a laser Doppler vibration meter
was integrated to obtain the displacement of the piezoelectric
deformation region 8 at this time. The percentages were plotted in FIG. 7
that represented changes in the displacement of the piezoelectric
deformation region 8 upon termination of specific driving cycles with
respect to the displacement in the initial state (0 cycle) before
starting the continuous driving.

[0165]The results showed that the displacement of the piezoelectric
deformation region 8 was remarkably reduced in the period up to the
10×108 cycle in the driving by the conventional pull-push
driving method, as shown in FIG. 7. On the other hand, the results
confirmed that the displacement was absolutely not reduced but slightly
increased to the contrary in the period up to the 20×108 cycle
at which the measurement was terminated in the driving by the driving
method according to the present invention.

(Voltage-Displacement Characteristic Test)

[0166]The liquid ejector 1 produced according to Example 1 was driven by
the driving method according to the present invention and the
conventional pull-push driving method with driving voltage waveforms
generated similarly to the above while varying applied driving voltages.
Then, displacements of the piezoelectric deformation region 8 of the
piezoelectric actuator 7 similarly to the above was measured. The driving
frequency was set to 2 kHz in both of the driving methods. The relation
between the value of the first voltage (-VL) [=the value of the
second voltage (+VL)] and the displacement of the piezoelectric
deformation region 8 in the driving method according to the present
invention and the relation between the voltage VH and the
displacement of the piezoelectric deformation region 8 in the
conventional pull-push driving method were plotted in FIG. 8.
Consequently, the results confirmed that the values of the first and
second voltages applied to the piezoelectric deformation region 8 in the
driving method according to the present invention for obtaining the same
displacement can be reduced to about half of the value of the voltage
VH applied in the conventional pull-push driving method, as shown in
FIG. 8.

(Measurement of P-E Hysteresis Characteristics I)

[0167]A P-E hysteresis loop showing the relation between the intensity of
electric field E (kV/cm) and the polarization quantity P (μC/cm2)
of the piezoelectric ceramic layer 6 was measured when a triangle wave
having a frequency of 100 Hz and an amplitude of -10 to +10 V or a
triangle wave having a frequency of 100 Hz and an amplitude of -20 to +20
V as models of the first and second voltages were applied to the
piezoelectric deformation region 8 of the piezoelectric actuator 7 of the
liquid ejector 1 produced according to Example 1. A ferroelectric
characteristic evaluation system FCE-HS2 manufactured by Toyo Corporation
was used for the measurement. Consequently, the results confirmed that
the P-E hysteresis loop can be remarkably reduced when the first and
second voltages are set to 10 V at which the intensity of electric field
E (kV/cm) of the piezoelectric deformation region 8 of the piezoelectric
actuator 7 is not more than 0.8 times of the intensity of the coercive
electric field Ec of the piezoelectric ceramic layer as compared with a
case of setting the voltages to 20 V at which the intensity of electric
field E exceeds 0.8 times of the intensity of the coercive electric field
Ec. Since the thickness of the piezoelectric ceramic layer 6 is 10 μm,
the intensity of electric field E (kV/cm) at the time of applying the
voltage of 10 V to the piezoelectric deformation region 8 of the
piezoelectric actuator 7 is 10 V/0.001 cm=10 kV/cm.

(Measurement of P-E Hysteresis Characteristics II)

[0168]FIG. 10 shows results obtained by measuring P-E hysteresis loops
showing the relation between the intensity of electric field E (kV/cm)
and the polarization quantity P (μC/cm2) of the piezoelectric
ceramic layer 6 at the time of applying a triangle wave having a
frequency of 100 Hz and an amplitude of -10 to +10 V to the piezoelectric
deformation region 8 of the piezoelectric actuator 7 of the liquid
ejector 1 produced according to Example 1 as a model of the first and
second voltages in the driving method according to the present invention
or a triangle wave having a frequency of 100 Hz and an amplitude of 0 to
+20 V thereto as a model of the voltage in the conventional pull-push
driving method similarly to the above. When the areas of the respective
P-E hysteresis loops were measured from FIG. 10, the results confirmed
that the area of the P-E hysteresis loop in the driving method according
to the present invention is 1.2 times, i.e., not more than 1.3 times of
the area of the P-E hysteresis loop in the conventional pull-push driving
method.

(Measurement of Crystalline State)

[0169]X-ray diffraction spectra at Bragg angles 2θ of 43 to
46° was measured when the liquid ejector 1 produced according to
Example 1 was continuously driven by 10×108 cycles by the
driving method according to the present invention and the conventional
pull-push driving method with driving voltage waveforms generated
similarly to the above, the piezoelectric ceramic layer 6 was taken out
from the liquid ejector 1 and a circular X-ray beam having a diameter of
100 μm was spot-applied to the surfaces of the active region 15 and
the inactive region 16 exposed by removing the individual electrode 10.

[0170]The C-axis orientations IC were obtained from the diffraction
peak intensities of the [200] planes and those of the [002] planes in the
X-ray diffraction spectra through the expression (1), while obtaining the
ratios of these C-axis orientations IC to initial values of C-axis
orientations IC previously measured as to the piezoelectric ceramic
layer 6 before assemble into the piezoelectric actuator 7 similarly to
the above.

[0171]Consequently, the results showed that the C-axis orientations
IC of the active region 15 was remarkably changed to 1.5 times of
the initial values and that of the inactive region 16 was 0.7 times of
the initial values and the crystalline states were changed when the
liquid ejector 1 was driven by the conventional pull-push driving method.
On the other hand, the results confirmed that the C-axis orientation
IC of the active region 15 was 1.04 times of the initial values and
that of the inactive region 16 was 1.07 times of the initial values to
remain generally unchanged and the initial crystalline states were
maintained when the liquid ejector 1 was driven by the driving method
according to the present invention.

Example 2

[0172]The liquid ejector 1 shown in FIG. 1 having a unimorphic
piezoelectric actuator 7 was produced similarly to Example 1, except that
the thickness of a piezoelectric ceramic layer 6 was set to 15 μm and
a pressurizing chamber 2 was formed to have a plane shape of 2.2 mm by
0.65 mm. The coercive electric field Ec of the piezoelectric ceramic
layer 6 was 17 kV/cm.

(Ejection Test)

[0173]When the driving voltage waveform (+VL=15 V, -VL=-15 V,
driving frequency: 1 kHz) shown in FIG. 1 was applied to one
piezoelectric deformation region 8 of the piezoelectric actuator 7 of the
liquid ejector 1 produced according to Example 2 for driving the
piezoelectric deformation region 8 by the driving method according to the
present invention, so that a corresponding nozzle 3 ejected ink droplets
under a condition of a head drop speed of 9 m/s. At the same time a
strobe was flashed after 120 μs from the application of the driving
voltage waveform for taking an image of ink droplets on a position of 1
mm from the distal end of the nozzle 3 to confirm that no noise vibration
was caused since only two ink droplets of ordinary sizes were imaged. A
similar image taken in relation to a nozzle 3 corresponding to a
piezoelectric deformation region 8 adjacent to the driven piezoelectric
deformation region 8 confirmed that no crosstalk was caused since no ink
droplets were imaged.

[0174]When the driving voltage waveform (VH=+30 V, driving frequency:
1 kHz) shown in FIG. 11 was applied to one piezoelectric deformation
region 8 of the piezoelectric actuator 7 of the liquid ejector 1 for
driving the piezoelectric deformation region 8 by the conventional
pull-push driving method so that the corresponding nozzle 3 ejected ink
droplets under a condition of a head drop speed of 9 m/s. At the same
time a strobe was flashed after 120 μs from the application of the
driving voltage waveform for taking an image of ink droplets on a
position of 1 mm from the distal end of the nozzle 3 to confirm that
noise vibration was caused since five ink droplets in total including two
ink droplets of ordinary sizes and three small ink droplets were imaged.
A similar image was taken in relation to a nozzle 3 corresponding to a
piezoelectric deformation region 8 adjacent to the driven piezoelectric
deformation region 8 confirmed that a crosstalk was caused since small
ink droplets were imaged.